Electronic postage meters have been developed with electronic accounting circuitry. Postage meter systems of this type are disclosed in U.S. Pat. No. 3,978,457 for Microcomputerized Electronic Postage Meter System and in U.S. Pat. No. 3,938,095 for Computer Responsive Postage Meter. The electronic accounting circuits include memory capability to store postage accounting information. This information includes, for example, the amount of postage remaining in the meter for subsequent printing. Other types of accounting or operating data may also be stored in the memory. The memory function in the electronic accounting circuits have replaced the function served in postage meter by mechanical accounting registers. Postage meters with mechanical accounting registers are not subject to many of the problems encountered by electronic postage meters. Conditions cannot normally occur in postage meters with mechanical registers that prevent the accounting for a printing cycle or which result in the loss of data stored in the registers. This, however, is not the case with electronic postage meters.
It has been recognized that conditions can occur in electronic postage meters where information stored in the electronic accounting circuits can be permanently lost. The lost data can result in a loss of information representing the postage funds stored in the meter. Since data of this type changes with the printing of postage and is not stored elsewhere outside of the meter, there is no way to recover or reconstruct the lost information. As a result, a user may suffer a loss of postage funds if the data in the electronic accounting circuit is lost. One condition which can cause permanent loss of accounting information in electronic postage meters is a failure of the electrical operating power supplied to the meter. Where a power failure occurs the operating voltage supplied to the accounting circuitry likewise fails and the information stored in the memory may be lost.
Systems have been designed to preserve information stored in electronic memory units when power fails. Examples of systems of this type are shown in U.S. Pat. No. 3,859,638 for a Non-Volatile Memory Unit With Automatic Standby Power Supply; U.S. Pat. No. 4,049,951 for Data Detection Apparatus; and U.S. Pat. No. 3,676,717 for Non-Volatile Flip-Flop Memory Cell. These systems, in part, involve sensing power failure and taking measures to insure data is not lost such as by employing an axilliary standby power supply or by loading the data into a non-volatile memory. Other U.S. patents which show systems to protect stored information are U.S. Pat. No. 3,801,963 for Method and Apparatus for Transferring Data from a Volatile Data Store Upon the Occurence of a Power Failure in a Computer; U.S. Pat. No. 3,959,778 for Apparatus for Transferring Data from a Volatile Main Memory to a Store Unit Upon the Occurence of an Electrical Supply Failure in a Data Processing System; U.S. Pat. No. 3,810,116 for Volatile Memory Protection; and U.S. Pat. No. 3,980,935 for Volatile Memory Support System.
Power failure protection systems have been incorporated in electronic postage meter systems. A postage meter power failure protection system is shown and described in U.S. Pat. No. 3,978,457 for Microcomputerized Electronic Postage Meter System. In this system, when a voltage drops below a threshold level, a signal is generated which initiates a shut down routine. As part of the shut down routine, the contents of a working random access memory are transferred to a non-volatile memory. The maximum time to detect the shut down signal and the time to transfer the register contents from the work memory to the non-volatile memory is a function of the circuit components, including the power supply filter capacitors. It is recognized that during "power-up" and "power-down" the microprocessor is not functioning predictably and that the memory must therefore be protected. The protection is accomplished by gates circuit. This system is very satisfactory and properly performs the desired function.